msm: camera: crm: protect unlink function with lock

Camera request manager need to handle the unlink IOCTL and
cam_req_mgr_cb_add_req simultaneously. Added link mutex protection in
the unlink and cam_req_mgr_cb_add_req functions. Added link state
check in the camera request manager cam_req_mgr_ops functions.
Without state check and mutex protection, cam_req_mgr_cb_add_req
may encounter link work queue use after free issue.

Change-Id: I062c0f7d6db842e4785aaac461a6132bcdabbe86
Signed-off-by: Ravikishore Pampana <rpampana@codeaurora.org>